There are numerous compounds that are used in the production of plastic components. Often, the components are formed using a molding process. To successfully manufacture a molded plastic part, a mold release agent is used in the manufacturing process to permit releasing the finished component from the mold.
One general class of mold release agent is "external" mold release agents. External mold release agents are applied directly to the mold. This procedure increases the manufacturing time and must be repeated every one to five parts. In addition, the mold release agent builds up on the mold, so the mold must be cleaned periodically with a solvent or washing agent. This is costly and time consuming.
A second general class of mold release agent is "internal" mold release agents. Internal mold release agents are added directly into the molding compound. Since they do not have to be continuously reapplied to the mold, internal mold release agents increase productivity and reduce cost.
There are many examples of compounds containing internal mold release agents for manufacturing various types of plastic components. Lee, U.S. Pat. Nos. 4,408,000 and 4,409,351 disclose the use of a fatty acid in the production of thermoplastic parts, which generally are flexible.
Various mold release agents are used in the production of other flexible elastomers, such as polyurethanes and other foams. Haas et al., U.S. Pat. No. 4,753,966 discloses the use of a zinc salt dissolved in an amine liquid as a mold release agent in the production of polyurethanes. Similarly, Meyer et al., U.S. Pat. No. 5,182,034 discloses the use of a metal salt combined with an amine as a mold release agent in the production of polyurethanes.
Mackey, U.S. Pat. No. 5,576,409 discloses the use of a fatty acid amide as a mold release agent. Haylock et al., U.S. Pat. No. 4,551,507 discloses a molding compound including a linear saturated polyester. The mold release agent is a metallic amine carboxylate formed from a metal-containing salt.
One type of molding compound, not described in any of the above patents, is an unsaturated polyester thermosetting molding compound. Such compounds sometimes are referred to as unsaturated polyester resin systems because they all include an unsaturated polyester resin. These compounds are well known in the art. For example, U.S. Pat. Nos. 5,585,439, 5,561,192, 5,449,549, 5445,877, and 5,412,003 each describe various formulations of unsaturated polyester resin systems, which differ from the present invention as described below. They are used in the production of strong, rigid plastic components used as automotive parts, boat hulls, other transportation products, appliances, furniture, computer components, electrical products--anywhere where it is desirable to have a strong, rigid, intricate plastic part manufactured to a stringent tolerance.
Two main types of unsaturated polyester thermosetting molding compounds are known as bulk molding compounds ("BMC") and sheet molding compounds ("SMC"). Such compounds include an unsaturated polyester resin. These resins, with a catalyst, can be cured in a heated mold to form a clear, hard composition. As it cures, the resin has a tendency to shrink slightly, thereby aiding in the release from the mold. The resultant composition, however, is extremely brittle, and therefore not useful in most applications. Therefore, a reinforcement component is added. A filler component, generally a fine ground mineral, is also added to improve appearance and surface characteristics. One common reinforcement material is chopped fiberglass. The resin, glass, and filler can be mixed directly to form a compound having a dough-like consistency, which is then added to a mold. Such a compound is commonly known as a bulk molding compound or BMC. Alternatively, as is known in the art, the molding compound can be formed into a thin sheet, called a sheet molding compound or SMC. The SMC is then molded. Kawakami et al., U.S. Pat. No. 5,445,877, which is incorporated here by reference, provides an example of the making of a sheet molding compound. Both BMCs and SMCs are particularly useful in manufacturing strong, intricate components to stringent tolerances. As, for example, in some of the systems described in the patents referenced above, other components commonly used in BMCs and SMCs include a low profile thermoplastic additive, a crosslinking monomer, and a thickener. Optionally, a pigment also can be included to produce a colored molded part. The use of such additives in unsaturated polyester thermosetting molding compounds, for both BMC and SMC processes, is well known in the art.
When mixed, the resin penetrates or wets the fiberglass reinforcement, thereby dispersing throughout the molding compound. It is desirable to have as much filler as possible in the BMC or SMC, since filler is far less expensive than resin. The addition of filler (and glass), however, inhibits the shrinking that occurs when the resin is molded by itself. BMCs and SMCs, therefore, require that a mold release agent be added. The more filler added, the harder it is to release the finished component, and the more mold release agent that must be used. Likewise, the more intricate the part, the harder it is to release, and the more mold release agent that must be used as well.
The most common mold release agent used in BMC and SMC processes is zinc stearate. It is estimated that zinc stearate is used as the mold release agent in as much as 90% of BMC and SMC processes. Zinc stearate acts by exuding to the surface of the molding compound, thereby contacting the mold and providing lubrication at the mold surface to permit release.
Zinc stearate is a fine, dusty powder, which creates several difficulties. As a powder, zinc stearate is difficult to disperse in the liquid resin. Thus, lumps can form. When lumps of zinc stearate exude to the mold surface, the result is imperfections or pitting on the surface of the finished product. These imperfections interfere with the subsequent treatment of the surface of the part, including interference with the application of vapor deposited metals, laminants, adhesives, paints and the like commonly applied to the surface of molded plastic parts. Depending on the part and its use, the part must be sanded, specially washed, or otherwise treated before adding surface coatings. For example, Uddin, et al., "Laser Surface Engineering of Automotive Components", Plastics Engineering (1997), discloses a method by which SMC automotive parts are treated with a laser to improve surface characteristics. Sometimes, imperfect parts must be discarded. The problems encountered using zinc stearate become more acute as the component becomes more intricate, since more mold release agent must be used.
Powder mold release agents such as zinc stearate also reduce the amount of filler that can be used in that they raise the viscosity of the liquid polyester resins. As a result, powder mold release agents increase the viscosity of the molding compound inhibiting the wetting of the reinforcement by the resin. Thus, more resin and less filler must be used, which increases raw material costs.
To overcome the drawbacks of powder mold release agents like zinc stearate, there has been a need for a liquid mold release agent for use in BMC and SMC processes. State-of-the-art liquid mold release agents are liquid zinc salts and phosphate esters. These materials, however, do not provide sufficient mold release activity for use in the low-shrink resin systems of BMC and SMC processes, and particularly are unsuitable for use in the molding of intricate parts to stringent tolerances. These materials also are very expensive. Thus, for the vast majority of BMC and SMC processes, powder mold release agents such as zinc stearate (and to a lesser extent calcium stearate) remain the primary currently available option.
Two other types of processes for molding with unsaturated polyester thermosetting molding compounds are pultrusion and wet-mat molding processes. In pultrusion, the reinforcement, usually fiberglass, is wheeled off a spool, dipped into a resin mixture, and pulled through a heated die to cure the compound. A mold release agent is used as part of the resin mixture to release the compound from the die. In wet-mat molding, a reinforcement mat is layed on the mold and the molding compound is poured on top of the mat. A mold release agent likewise is used in wet-mat processes. Pultrusion and wet-mat molding compounds differ from BMCs and SMCs partly in that no low profile thermoplastic additive or thickening agent is used. BMCs and SMCs are preferred for more complex molded components, and generally are more difficult to release.
The inventors of the present invention have discovered that certain tertiary alkyl primary amine carboxylates are soluble in and highly compatible with uncured unsaturated polyester resins thereby eliminating many of the disadvantages of zinc stearate and state-of-the-art liquid mold release agents, and are more efficient as mold release agents. The liquid mold release agents of the present invention are particularly advantageous in BMC and SMC processes where release is more difficult to achieve.